LAS VEGAS — The White House campaign has brought a new act to Vegas. Barack Obama has stepped up his campaign against Hillary Rodham Clinton, and he’s trying to use humor to bring her down before this weekend’s Democratic presidential caucus. His argument is starkly different from the ‘‘Iowa nice’’ approach he used in recent weeks when campaigning in the first caucus state. Candidates who go negative there have a history of turning off voters, so Obama rarely criticized Clinton directly in Iowa — instead he made veiled references to ‘‘some of my opponents’’ — and he won the state. But there’s nothing subtle about Las Vegas. With a high-stakes match on the line Saturday, Obama embraced local traditions by debuting a biting political standup routine Thursday night that mocked his rival. Obama began by recalling a moment in Tuesday night’s debate when he and his rivals were asked to name their biggest weakness. Obama answered first, saying he has a messy desk and needs help managing paperwork — something his opponents have since used to suggest he’s not up to managing the country. Former North Carolina Sen. John Edwards said his biggest weakness is that he has a powerful response to seeing pain in others, and Clinton said she gets impatient to bring change to America. ‘‘Because I’m an ordinary person, I thought that they meant, ‘What’s your biggest weakness?’’’ Obama said to laughter from a packed house at Rancho High School. ‘‘If I had gone last I would have known what the game was. And then I could have said, ‘Well, ya know, I like to help old ladies across the street. Sometimes they don’t want to be helped. It’s terrible.’’’ ‘‘Folks, they don’t tell you what they mean!’’ he said. Obama chuckled at his own joke before riffing on another Clinton answer in the debate, when she said that she is happy that the bankruptcy bill she voted for in 2001 never became law. ‘‘She says, ’I voted for it but I was glad to see that it didn’t pass.’ What does that mean?’’ he asked, again drawing laughter from the crowd and himself. ‘‘No seriously, what does that mean? If you didn’t want to see it passed, then you can vote against it! People don’t say what they mean. ‘‘You know what I’m saying is true,’’ he said, then addressed his routine directly at audience members who don’t know who they will vote for yet. ‘‘Undecideds, remember now, remember what I’m saying.’’ He continued by responding to a new Clinton radio ad that accused him of having financial ties to supporters of the proposed Yucca Mountain nuclear waste site that most Nevadans are loath to come to their state. ‘‘I have said over and over again I’m against Yucca,’’ Obama said. ‘‘I’m against Yucca Mountain. I think the science is not there. I’ve never, I’ve never been for Yucca. Never been for it. Never said I was for it. ‘‘Suddenly you’ve got the Clinton camp out there saying, ‘He’s for Yucca.’ What part of I’m not for Yucca do you not understand?’’ he said, then laughed along with his audience. As the laughter subsided, Obama drove home the broader point he’s been trying to make against Clinton the entire campaign. ‘‘Those kinds of tricks, that kind of approach to politics is what has to stop because what happens is then nobody believes anything,’’ Obama said. ‘‘The voters don’t believe what politicians say. They get cynical. Folks in Congress, they’ll tell you they’re looking out for you — they’re looking out for somebody else. We have to change that politics and that’s why I’m running for president.’’ Clinton spokesman Phil Singer retorted with a dig at Obama’s experience. ‘‘Judging from his act, it doesn’t sound like Senator Obama has much experience doing stand-up either,’’ Singer said in a statement. ‘‘Nothing says change more than using jokes to distort his opponents’ records or to gloss over his vote against capping credit card interest rates or to dodge explaining why he didn’t speak out against the Yucca project when given the opportunity to do so.’’

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